The invention relates to a sealing blade, which includes opposite elongated edges and between them a flat and which is mountable to a blade holder arranged in a coating applicator of a coating device, where the sealing blade is brought into contact with a moving surface.
The coating or surface sizing of paper or board may be performed for example using a film transfer coater. In it, the web to be treated runs through a nip located between two rolls. The coating mixture or sizing agent is spread using a specific coating applicator onto the surface of one roll or both rolls, from which surface it is transferred onto the surface of the web to be treated in the nip between the rolls. The thickness and profile of the sizing agent layer on the surface of the roll may be adjusted for example by means of a doctor blade or a rotating metering rod.
The coating applicator may comprise an application chamber, which is limited in the direction of rotation of the roll by a doctor element and on the other hand by a front wall. In the lateral direction, the application chamber is limited by side seals. The coating mixture or surface sizing agent is fed into the application chamber for example through a channel located in a girder of the coating applicator. The coating mixture is fed into the application chamber at such a great speed and pressure that a return flow is created in the gap between the front wall and the application roll, in other words a coating mixture flow or sizing agent flow in a direction opposite to the direction of rotation of the application roll is created. The purpose of the return flow is to prevent the penetration of air, which is on the surface of the application roll, into the application chamber. If air can enter the application chamber, places with no coating may be created in the web.
It is well known that a sealing blade may be used as the front wall that closes the application chamber. The sealing blade may be pressed against the application roll so that no air escapes from between the blade and the application roll into the application chamber. There is no actual return flow from between the sealing blade and the application roll, but some coating mixture is led from the application chamber to the front of the blade through flow openings in the sealing blade. In this case, a pool of lubricant is formed in front of the sealing blade, which pool of lubricant prevents the friction occurring between the sealing blade and the application roll from damaging the roll or the sealing blade. If the pressure of the application chamber rises for example as a result of an excessive feeding of coating mixture, the force that presses the sealing blade against the application roll increases, whereby the sealing blade wears more rapidly and may damage the application roll or at least wear it.
However, a drawback related to the prior art sealing blade is that the edge of the blade placed against the application roll becomes sharp in connection with the use of the blade. This causes occupational safety problems in the maintenance of the coating applicator. A sharpened sealing blade is dangerous to the maintenance personnel for example when the sealing blade, metering rod or the cradle of the metering rod is replaced. For the replacement of just the metering rod and its cradle, which replacement needs to be performed in some cases more frequently than the replacement of the sealing blade, it is not often customary to remove the sealing blade from the coating applicator, because this is troublesome to perform. What makes the removal of the sealing blade difficult is that it may be firmly stuck to its blade holder and would therefore require washing to facilitate the removal. A sealing blade which has become sharpened and which is also difficult to remove may cause incised wounds to employees and also damage to the surface of the application roll.
Attempts in many different ways have been made to solve the problem related to the sealing blade becoming sharpened, but with little success. Since the holder of the sealing blade is fastened to the girder in a stationary manner, it is difficult to change the position of the holder. The sealing blade may also be lifted higher in the blade holder, but this does not always give the desired outcome, either. An attempt to enhance the lubrication that decreases the friction between the sealing blade and the application roll may be made by changing the feed pressure, but this is restricted by the seals that do not seal sufficiently well and by fouling problems. Moreover, it is not desirable to shift to a thinner sealing blade and a higher chamber pressure because of reasons such as increased leaks and splashes.
One prior art sealing blade that can be mentioned is in FI patent number 103058. The sealing blade known from it is a planar item. Both flat surfaces of the blade are hence essentially similar planar surfaces, in other words the front edge and the back edge of the blade are in the same plane.